One of the major problems in the assembly of an electret transducer is obtaining the correct tension in the transducer membrane diaphragm. The diaphragm tension greatly influences the transducer's frequency of resonance and acoustic sensitivity. A higher tension produces a higher resonant frequency, while a lower tension produces a greater sensitivity. Thus, the diaphragm tension is critical.
Several ways are taught in the prior art for obtaining the desired tension. A first approach, illustrated in Wilson U.S. Pat. No. 2,086,107 dated July 6, 1937, involves the use of a threaded lock ring to peripherally clamp the diaphragm within the transducer housing in a flat condition with essentially zero tension. A back plate that engages the central portion of the diaphragm is thereafter advanced in a controlled manner, such as by another threaded member, to apply a selected tension to the diaphragm.
Another approach, illustrated in Madsen et al U.S. Pat. No. 3,663,768 dated May 16, 1972 and Fraim et al U.S. Pat. No. 3,816,671 dated June 11, 1974, comprises applying the desired tension to the diaphragm by some means external to the transducer. The diaphragm is then joined to the back plate while under tension. This is accomplished by either mechanically clamping the diaphragm to the back plate as disclosed in Madsen or bonding the diaphragm to the back plate as disclosed in Fraim.
A still further approach, illustrated in Victoreen U.S. Pat. No. 3,814,864 dated June 4, 1974, consists of first affixing the diaphragm to a peripheral support member while under tension. This combination and the back plate are then assembled, and the back plate is further adjusted relative to the diaphragm support to achieve the final tension.
All of the foregoing arrangements require a fairly large number of assembly steps and/or some fairly elaborate fixturing in order to place the diaphragm under the desired tension. This necessarily increases the cost of manufacturing the transducer unit.